The integration of hundreds of millions of circuit elements, such as transistors, on a single integrated circuit necessitates further dramatic scaling down or micro-miniaturization of the physical dimensions of circuit elements, including interconnection structures. Micro-miniaturization has engendered a dramatic increase in transistor engineering complexity, such as the inclusion of graded well-doping, epitaxial wafers, halo implants, tip implants, lightly doped drain structures, multiple implants for source/drain regions, silicidation of gates and source/drains, and multiple sidewall spacers, for example.
The drive for high performance requires high speed operation of microelectronic components requiring high drive currents in addition to low leakage, i.e., low off-state current, to reduce power consumption. Typically, the structural and doping parameters tending to provide a desired increase in drive current adversely impact leakage current.
Recently, metal gate electrodes have evolved for improving the drive current by reducing polysilicon depletion. Simply replacing high resistance polysilicon gate electrodes with metal gate electrodes may engender issues such as, increased leakage current because of an undesired value of the work function which, in turn, provides undesired electrical characteristics for the transistor. The work function is the amount of energy required to excite electrons across a threshold. Polysilicon gates on silicon substrate provide a work function that allows the gates to be adequately controlled. The work function of n+ and p+ polysilicon is near to either the conduction or valence band and cannot be altered. The metal gate technique makes it feasible to modify the threshold voltage of micro-miniaturized CMOS device without performing adjustment implants which become difficult with aggressive scaling of the CMOS channel length. However, the use of a metal gate electrode on a silicon substrate undesirably alters the work function vis-à-vis polysilicon, thereby reducing the controllability of the gate.
Accordingly, a need exists for methodology enabling the fabrication of micro-miniaturized semiconductor devices comprising transistors with metal gate electrodes having a tunable work function.